Thin-film solar cells of the above-described type have possibilities for extensive practical use, as described in U.S. Pat. No. 4,335,226 (issued to Michelsen et al. on Jun. 15, 1982).
Such thin-film solar cells having a high conversion efficiency are disclosed in, e.g., U.S. Pat. Nos. 4,798,660 (issued to J. H. Ermer et al.) and 4,915,745 (issued to G. A. Pollock et al.); C. L. Jensen et al., Proceedings 23rd Photovoltaic Specialists Conference, (1993) p. 577; and JP-A-4-326526 (Mitsune et al.). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
C. L. Jensen et al. suggested in the reference cited above that the CuInSe.sub.2 (hereinafter referred to as "CIS") light absorbing layer formed by heating a stacked precursor film comprising a copper-gallium alloy layer containing gallium and an indium layer in a selenium atmosphere has improved adhesion to a metallic back electrode layer of molybdenum (Mo) because of the segregation of gallium and migration thereof to the metallic back electrode layer during the heat treatment, and that there is the possibility that the light absorbing layer formed has an internal structure having a gallium concentration gradient and made up of two layers, i.e., a CIGS layer and a CIS layer, based on the results of Auger electron spectroscopy (AES).